Would the person be one of your parents? A relative who has taken you under his wing? Or would it be your junior high school teacher whose words of encouragement still echo in your ears?
While parents, relatives and teachers can be terrific cheerleaders for encouraging you to develop your strengths, the person who should believe in you more than anyone else should be YOURSELF! Self-belief (not haughtiness) is essential for developing your potential to its fullest.
If you have even the slightest self-belief doubt, get your mind on your side by building your self-confidence. It is a learnable trait!